This invention relates to the fabrication of large scale integration (LSI) and very large scale integration (VLSI) circuit chips, and more particularly to an automated high-speed electron beam system which is used as a tool in the fabrication of such chips. Even more particularly, the invention relates to a process whereby electron beam systems can be made compatible with each other, allowing more than one electron beam system to be used in the fabrication of the same chips.
When an integrated circuit chip is fabricated, it is one of many chips on a wafer of semiconductor material. Prior to each step in the fabrication process, the wafer is coated with a photographic type material called a resist. The resist is then exposed, by one of a variety of techniques, with an integrated circuit pattern corresponding to the next step of the process.
After being exposed, the resist is developed, uncovering those areas of the wafer that are to be subjected to the next step of the process and protecting those areas that are not to be affected. When the fabrication process is complete, the wafer is scribed along the unused channels between the chips and the individual chips are broken off from the wafer.
As integrated circuit technology has improved, the dimensions of the individual circuits on a chip have decreased, allowing a higher density of circuits per unit area. The higher circuit density, along with improved fabrication techniques and a desire for more functions per chip, have resulted in what is now called LSI and VLSI. The higher circuit densities associated with LSI & VLSI, as well as the larger chip size that has resulted from improved fabrication techniques, have required that the circuit patterns used for each step of the fabrication process be made more accurately so as to align with each other. The high resolution of an electron beam system makes it a good tool for generating the required circuit patterns.
When an electron beam system is used to generate the circuit pattern of a wafer, there are two basic ways in which the exposure can be made:
(1) expose the desired pattern directly on the resist coated wafer; and (2) create a mask that has the desired pattern.
The direct wafer exposure has the advantage of greater resolution since an electron beam, rather than light waves, is being used to make the exposure. However, it has the disadvantage of requiring that the wafer be removed from the electron beam system after each exposure for the next step of processing and then, after processing, be precisely aligned within the electron beam system for the next exposure.
When masks are used, a glass plate, which has had a metal film deposited on it and a resist over the metal film, is exposed with the desired circuit pattern by the electron beam system. After developing, the metal film that remains on the glass plate is the circuit pattern for one process step for one chip. This pattern is typically made at a 10:1 size and is called a reticle. A separate reticle is required for each step of the fabrication process. The reticle is used in a direct-step-on-wafer (DSW) system which reduces the pattern by a factor of ten and exposes it on the wafer. The DSW system then steps the pattern to the next chip location on the wafer and exposes it. This process is repeated until every chip location on the wafer has been exposed with the desired pattern.
The area over which an electron beam can be accurately deflected, called the field, is typically several hundred microns square. This is very small compared to the size of the chip. To overcome this limitation, the chip circuit pattern to be exposed is divided into the appropriate number of scan fields, each not exceeding the field size of the electron beam system.
The device to be exposed (wafer or glass plate) is mounted on a stage which is movable in both the X and Y direction. The stage is positioned at the first scan field location and all the circuit patterns of the scan field are exposed by the electron beam system. The electron beam is then turned off, or blanked, and the stage is moved so that an adjacent scan field of the circuit pattern is aligned under the electron beam. The electron beam is then turned on and the circuit pattern within that scan field is exposed. This process is repeated until the entire circuit pattern has been exposed.
Electron beam systems are typically controlled by a computer. The circuit pattern to be exposed consists of a large number of rectangles; even a line is a rectangle, since it has a length and a finite width. Irregular shapes, if any, can be approximated by exposing a group of small rectangles. The data specifying the rectangles which represent the circuit pattern and the data specifying the stage positions are stored in either the computer's memory or in some storage device external to the computer until it is ready to be sent to the electron beam system.
The computer program causes this data to be transmitted to the electron beam system controller which uses it to move the electron beam over all the rectangles in the scan field. The stage is then positioned to an adjacent scan field, with the beam blanked. This scan field is exposed, and so on. Exemplary prior art electron beam systems are described in U.S. Pat. Nos. 4,132,898 and 4,147,937.
The position of the electron beam within the field is controlled electromagnetically by controlling the current supplied to the deflection coils. The amount of current required to deflect the beam a given amount can be precisely determined and the electronic circuitry that provides the control can be designed accordingly.
Two deflection coils, one for the X direction and one for the Y direction are used to position the electron beam anywhere within the field. The XY coordinate system resulting from the two coils does not have truly perpendicular axes because of the physical impossibility of mounting the coils at exact right angles. The deflection coordinate system will also be rotated some amount from the coordinate system of the stage. Calibration procedures have been devised to electronically compensate the current supplied to the deflection coils to correct for such perpendicularity and rotational problems.
The position of the stage, on which the device to be exposed is mounted, is controlled by a servo system using X and Y coordinates supplied by the computer program. Two mirrors are mounted on the stage, one in the X direction and one in the Y direction. A laser beam is reflected off these mirrors, by means of an interferometer, to an X receiver and Y receiver. These receivers convert the reflected laser beams to electrical signals that are proportional to the motion of the stage. These electrical signals, in turn, are fed back to the servo controller which uses them to control the position of the stage. The mirrors determine the X and Y axes of the stage coordinate system, and just as the deflection coils, can not be mounted exactly perpendicular to each other.
As mentioned previously, when all of the pattern in a given scan field is exposed, the stage is used to position an adjacent scan field under the electron beam so that another pattern can be exposed. If the coordinate system of the stage is not perpendicular, and is rotated with respect to the coordinate system of the electron beam, then the exposed scan fields will not truly butt up against each other. In some cases, the scan fields may overlap and in others there may be a gap between them.
Since the circuit patterns being exposed cross the boundaries of the scan fields, this would result in wafers or masks that are not usable. Calibration procedures are known in the art that measure the perpendicularity and rotation of the stage coordinate system. The computer program uses these measured values to determine the constants k for a set of linear transformation equations. Whenever the stage is to be positioned to a new scan field location at a specified coordinate X,Y, the computer program transforms the coordinates to X',Y', using the linear transformation equations. The coordinates X',Y' are sent to the servo system and compensate for perpendicularity and rotation problems.
Thus, techniques are known in the art to correct the perpendicularity and rotation problems of the two coordinate systems involved in an electron beam exposure system. Unfortunately, these techniques still have positional errors associated with them as large as several microns over a distance of several milli-meters. However, as long as all masks or wafers are exposed on the same electron beam system, they all have the same minor distortion, and they can be used in the fabrication of LSI and VLSI circuits.
The positional error referred to in the preceeding paragraph is primarily caused by the fact that the mirrors mounted on the stage are not absolutely flat and distort the feedback signal to the servo controller. Unfortunately, glass plates, exposed on two different electron beam systems, are not compatible, even if all the perpendicularity and rotation corrections were made on each system, since the positional error caused by the mirror distortion of the two systems may cause pattern defects at different coordinates.
As described above, it should be evident that a significant drawback of prior art electron beam systems, when used as a tool in the fabrication of LSI and VLSI circuits, is their noncompatibility due to positional and other errors. For example, the "master slice" technique is commonly used in LSI and VLSI technologies. With this technique, the process steps which form the circuit elements of the chips are the same for every wafer. Wafers are fabricated, using a set of masks that form the master slice, and are stockpiled. Several additional process steps add the wiring, or metalization, that interconnects the circuits and causes the chip to perform the desired function. Because of positional errors, the masks which add the wiring must be made on the same electron beam system as those that were used to fabricate the masks of the master slice. This greatly limits the output capability of a wafer processing facility. Further, if an electron beam system has to have a mirror replaced, the replaced mirror will have a different distortion, and subsequent masks produced will not be compatible with previous masks. Since the electron beam system is a production tool used in the fabrication of VLSI masks, and since (as a production tool) it is desirable to increase wafer, mask, and/or interconnect throughput, it is seen that there is a great need in the art for compatible electron beam systems.